Aluminum-zinc alloys



Patented Feb. 1, 1939 UNITED STATES PATENT OFFICE ALUMINUM-ZINC ALLOYS George F. Comstock, Niagara Falls, N. Y., assignor to The Titanium Alloy Manufacturing Company, New York, N. Y.,' a corporation of Maine No Drawing.

Application February 18, 1937.

Serial No. 120,306

Claims. (Ci. 25-145) My invention relates generally to aluminumzinc alloys in which aluminum is the major constituent, and particularly to complex light alloys containing chromium, titanium and magnesium 5 as well as zinc and aluminum, besides the usual expensive quenched and tempered aluminum alloy castings. The fact that no quenching treatment, or even heating, is required for developing high strength in my improved aluminum-zinc alloys is an important advantage. This alloy is also readily machinable and highly resistant to corrosion.

Although the examples and test-results hereinafter described apply to castings, it is obvious that my improved aluminum-zinc alloy, which does not require heat-treatment to develop'high strength, would also be highly useful in wrought form. Hence my invention is not confined to castings alone.

Aluminum-zinc alloys have also been highly recommended for their excellent mechanical properties, but in order to obtain tensile strengths as high as 30,000 pounds per square inch in sand castings, the zinc content must be up to about Such zinc content has been found in practice to be too high for good casting properties, strength at high temperatures, or corrosion resistance, so that these plain aluminum-zinc alloys have not been very much used in industrial practice. Substantially the same objections apply when part of the, zinc is replaced by copper, which, although improving the stiffness or yield point appreciably, impairs the ductility rather seriously.

According to my invention, the zinc content is held to about 10% or less, and the hardening and strengthening effects of chromium, titanium, and magnesium are utilized to obtain a superior combination of yield point, strength, and ductility. These last-mentioned elements all improve the corrosion resistance also, and in'addition the titanium is advantageous in refining the grain, while the magnesium promotes age-hardening at room temperature.

The preferred composition for my new alloy is about 8% zinc, 0.5% chromium, 0.15% titanium, 0.8% magnesium, less than 0.8% iron, and less than 0.3% silicon, copper or manganese, with balance substantially aluminum.

Permissible limits in composition within which the properties do not vary seriously are about as follows:

I Per cent Zinc. 5 to 10 Chromium .2 to .6 Titaniu .1 to .35 Magnesium .3 to 2.5

Balance substantially aluminum.

The magnesium content should vary inversely with the zinc, since when both are low, the yield point is low, and when both are high the ductility is low.

To illustrate this relation, the following tensile tests results are set forth, each value being the average of two tests (with very few exceptions, due to flaws), made on castings not heat-treated but merely aged about four days at room temperature.

ultimate tensile Table 1 Percent added Lbs. per sq. in.

- Percent Heat No. Yi m T u elgingae ens e on Zn or T1 Mg point strength 0. 47 0. l5 0 17290 21300 2. 0 4 1 0 17250 23200 4. 0 .49 26 0 11900 23700 11. 3 5 l5 0 11250 24000 12. 0 5 l6 2 19500 28800 5. 5

. 5 l5 3 23400 32500 4. 7 5 15 6 27000 32600 2. 5 5 15 4 24200 32700 5. 2 45 l5 6 25000 33400 3. 8 5 15 s 8 27900 35300 4. 5

Comparing heat '7 in Table 1 with heats 16 and 17, it will be seen that 'as much as 0.6% magnesium gives a rather low elongation with 10% zinc, but with 5% zinc, 1 or 2% magnesium may be added without reducing the elongation to an equally low value. The yield point is seen to be quite low in heats 1 to 5 with low magnesium, or in heat 16 with low zinc; but taking heat 16 as a starting point, it was raised appreciably either by increasing the zinc as in heats '7 and 10, or by increasing the magnesium as in heat 17:" The best combinations or yield point and strength with reasonable elongation were obtained in heats 9, 10 and 12 with 7.5 to 8.5% zinc and 0.6 to 0.9% magnesium, the yield points being 25000 or more, the tensile strength over 33000, and the elongation at least 3.8%.

The best proportions oi chromium and titanium in my improved alloys were determined by some preliminary experiments which indicated.that with more than about 0.5% chromium, the alloys were embrittled so that both strength and with 3 and 4, shows that higher chromium content reduced both strength and ductility with only a slight improvement in yield point. Sample 23 is very similar to '7, showing that 0.25% chromium is almost as effective as 0.5%; but sample 24. when compared with 9, shows that in the absence of chromium the tensile properties are inferior.

It is considered advisable in these alloys to keep the chromium content close to 0.5% so as to secure the maximum benefit in resistance to corrosion without impairment of physical properties.

The addition of silicon, copper, or cadmium to these alloys is detrimental to the tensile properties of castings as illustrated by the tests reported in Table 3. Silicon was added as calcium silicide, containing about 65% silicon, except in one instance when 90% ferrosilicon was used with inferior results.

Table 3 Percent added Lbs. per sq. in. Percent Heat No. 1 M T u elangae ens e on Or Ti Mg Si Cu d point "numb 0. 0. 15 0. 5 0 0 0 27000 32000 2.5 5 l5 5 3 0 0 27200 30700 2. 2 .5 15 .6 .3 (FeSi) 0 0 25100 27200 2.2

ductility were-reduced. Likewise I'found that no particular advantage was obtained with over 0.15% titanium, although 0.1% was barely suflicient. These relations are illustrated by the following results:-

Table 2 Percent added Lbs. per sq. in.

Pleroent Heat No. e onga- Zn or T1 M Yield Tensile tion 5 point strength The effect of variations in titanium content is shown by the first five samples listed in Table 2, in the comparison between samples 22 and '7, and by the last Iour samples listed. The addition of about 0.15% titanium produced a decided improvement in strength and yield point, and the grain size of the castings without titanium could be observed plainly'to be coarser than the grain size of those containing titanium.

The eilect oi. variations in chromium is also shown in Table 2. Sample 21, when compared The first seven tests listed in Table 3 show that silicon decreases the strength 01 these alloys, with small irregular eilects on the yield point and elongation. The addition or iron as in heat 29, which showed 1% iron on analysis, had a further detrimental effect. Heat 33 as compared with 32, and heat 34 as compared with 9, show that copper seriously decreased both strength and elongation, but with an increase in yield point. Cadmium as shown by the last four tests listed in this Table 3, whether substituted for aluminum or for zinc in the alloys, decreased the strength and yield point with an improvement in ductility.

It is evident therefore that a good grade of aluminum, low in silicon and copper, should be used for making my improved high-strength aluminum casting alloy. Iron up to 0.8% does not seem to be detrimental. The aluminum for these experiments was of at least 99% purity, and was melted in clay-bonded graphite crucibles in a gas-fired furnace. When the melt had reached a temperature of 1425" F., the chromium and titanium were added, either in the form 01' separate aluminum alloys containing respectively 6 to 7% chromium, and 6 to 7% titanium, or in the form oi a single master alloy containing about 2.9% titanium and 8.5% chromium.

This last mentioned. master alloy may be made by adding gradually to molten aluminum superheated to 2250 F. a mixture of green chromic oxide, pure white titanium oxide, and cryolite in about the following proportions: For 9 lbs. aluminum, I used lb. T102, 1% lb. CrzOa and 2 lbs. cryolite. Each small addition of the mixed oxides and flux should be thoroughly stirred into the made by these methods have contained 2 to 4% titanium and 8 to 10% chromium. After the titanium and chromium are added ,to the alumlnum at about 1425 F. in the production of my new high strength alloy, and are fully dissolved,

the zinc is added as the pure metal, stirred well, and finally the magnesium is added in the same way.

The molten alloy should be fluxed with a little zinc chloride, stirred, skimmed, and allowed to cool to a suitable temperature for pouring the castings desired. For cast-to-size tensile test bars in dry sand molds, the pouring temperature was about 1300 F.

This new aluminum-zinc alloy can be remelted repeatedly without serious deterioration, provided. its composition is maintained substantially constant. With careful melting practice, small additions of magnesium should be the only adjustment required. Sodium has also been used as a preventive of oxidation losses on remelting, but magnesium I have found to be preferable.

Some results obtained on remelting are presented below:

later it is highenwithout much change in ductility. Examples 01. these changes on ageing I will now describe:

Table 5 Per cent added Lbs. per sq. in. Per cent Yield Tensile tion days Zn or T1 Mg point strength 7 3 l0 5 .15 i 0 27000 3x00 2. 5 7 16 10 .5 .15 .0 31500 35200 2.3

- It is evident from Table 5 that yield points above 30,000 1bs. per sq. in. with tensile strengths over 35,000 are obtainable in these cast alloys after ageing two weeks at room temperature, with as much elongation as is shown by the com- Table 4 Original alloy charge 3532 Lbs. per sq. in. Times Per cent Heat melted Y1 m T n elongation 0 one a I Zn Cr T1 Mg Si Na Mg point strength 1 10 0. 5 0. i5 0. 6 0. 3 27200 a 30700 a 2 3 10 5 15 0 3 l 0 20800 33300 3. 5

I might state that heat 39 as given in Table 4 is really an average of heats 6 and 7 (Table 1), both of which were remelted together to obtain heats 40 to 43 inclusive. Heat 44 is likewise an average from heats 8 and 10, which were simimon aluminum casting alloys of about 22,000 lbs. per sq. in. tensile strength. It more than 3% elongation is required after ageing, the zinc content should not be as high as 10%, nor the magnesium over 1%, and the yield point then will.-

not be quite up to 30,000 lbs. per sq. in.

The various alloys described above have been distinguished on the basis of the materials added in the melting crucible. Owing to melting losses or absorption of impurities, the compositions of the'alloys tested may have been somewhat different To obtain an idea of the degree 01' these diiferences, I'made some chemical analyses 01. the test bars, and the results are shown in Table 6.

Table 6 Per 0011000000 Chemical analysis HOStNO.

Zn Cr h Mg .Zn Cr Ti Mg Fe 81 Cu Mn 8.5 5 .15 .0 8.08 .49 .172 0.05 0.75 s .15 .s 7.04 .50 .105 .74 .0 0.1 0.07 0.03 s 5 .15 .75 7.01 .40 .135 .00 12.5 .4 .1 0 .43 .238 10 '11s .15 .95 10.21 .40 .105 .02 .70 p15 .08

Theironcontentswsreioundtohavebeenderived from the'aluminum ingots used as a basis for these experimental alloys. This lot of ingot contained 0.49% iron. Although that amount of chromium, titanium, or magnesium would have an important eil'ect on the properties of these alloys, iron is much less powerful in its influence,

and may be present in the alloys in any amount,

from the usual low value of about 0.3% up to 0.8% without making much difference.

I claim as my invention:

1. An alloy consisting of about 80% to 95 aluminum, and from about 3 to 15% zinc with the remainder composed of small proportions of chromium to about 0.75%;01 titanium to about 0.35%, and of magnesium to about 2.5%.

2. An alloy consisting of about 80% to 95% aluminum, from about 5% to 12% zinc, from about 0.20% to 0.75% chromium, from about 80 0.10% to 0.35% titanium, and from about 0.30% to 2.5% magnesium.

auasso 3. An alloy consisting oi from about 5% to 12% zinc, from about 0.20% to 0.75% chromium, from about 0.10% to 0.35% titanium, and from about 0.30% to 2.5% magnesium with the balance being aluminum except for the normal impurities round in aluminum. 4. An alloy consisting oi. from about 5% to 12% zinc, from about 0.10% to 0.75% chromium, from about 0.10% to 0.35% titanium, from about 0.30% to 2.5% magnesium, not over 0.80% iron, not over 0.30% each of silicon and copper, and; the balance commercial aluminum having impurities not in excess of 1.0%.

5. An alloy consisting 01 about 8% zinc, 0.5% chromium, 0.15% titanium, 0.80% magnesium, with traces up to 0.80% iron, with traces up to 0.30% each of silicon and copper and the bal ance commercial aluminum having impurities not in excess of 1.0%.

GEORGE r. comsrock. 

